Several Local Corrosion Characteristics of Titanium Materials

 1.1 Crevice corrosion The crevice corrosion resistance of titanium is particularly strong, and crevice corrosion occurs only in a few chemical media. The crevice corrosion of titanium is closely related to temperature, chloride concentration, pH value and the size of the crevice. According to relevant information, crevice corrosion is easy to occur when the temperature of wet chlorine gas is above 85 ℃. For example, in some factories, a packed tower is used to directly cool the wet chlorine gas before the cooler to reduce the temperature of the wet chlorine gas to 65-70 ℃, and then enter the titanium cooler to improve the resistance to crevice corrosion, and the effect is also very significant. Practice has proved that reducing the temperature is one of the effective methods to prevent crevice corrosion, and crevice corrosion of Grade 5 Ti-6Al-4V Titanium Wire has also occurred in high temperature sodium chloride solution. In short, for the parts and components that are prone to crevice corrosion, such as the sealing surface, the expansion joint between the tube sheet and the tube, the plate heat exchanger, the contact part between the tray and the tower body, and the fasteners in the tower, titanium such as Ti-0.2Pd should be used. Alloys, gaps and stagnation areas should be avoided in the design. For example, the fasteners in the tower should be connected with bolts as much as possible. It is better to use expansion joint and sealing welding structure for tube sheet and pipe than simple expansion joint. For flange sealing surface, asbestos gasket should not be used, but PTFE film-coated asbestos gasket should be used.

1.2 High temperature corrosion

The high temperature corrosion resistance of Grade 5 Ti-6Al-4V Titanium Sheet depends on the characteristics of the medium and the performance of its own surface oxide film. In air or oxidizing atmosphere, titanium can be used as a structural material up to 426 °C, but at about 250 °C, titanium begins to absorb hydrogen significantly. In a complete hydrogen atmosphere, when the temperature rises above 316 °C, titanium Hydrogen absorption becomes brittle. Therefore, without extensive testing, titanium should not be used in chemical equipment with a temperature higher than 330 °C. Considering hydrogen absorption and mechanical properties, the operating temperature of all-titanium pressure vessels should not exceed 250 °C. The upper limit of the use temperature of the tube is about 316°C.

1.3 Stress corrosion

Except for a few kinds of media, industrial pure titanium has excellent stress corrosion resistance, and the phenomenon of titanium equipment damage caused by stress corrosion is still rare. Industrial passive titanium is only used in fuming nitric acid, some methanol solutions or some hydrochloric acid solutions, high temperature hypochlorite, molten salt with a temperature of 300-450 ° C or NaCl atmosphere, carbon disulfide, n-hexane and dry chlorine and other media. Stress corrosion occurs. The tendency of stress corrosion cracking of titanium in nitric acid increases with the increase of NO2 content and the decrease of water content. The stress corrosion tendency of titanium reaches its maximum in anhydrous nitric acid containing 20% ​​free NO2. When the concentrated nitric acid contains more than 6.0% NO2 and less than 0.7% H2O, the stress corrosion cracking of industrial pure titanium occurs even at room temperature. Serious stress corrosion and explosion have occurred in my country when titanium equipment is used in 98% concentrated nitric acid. Industrial pure titanium is sensitive to stress corrosion cracking in 10% hydrochloric acid solution, and titanium produces stress corrosion in 0.4% hydrochloric acid plus methanol solution.

To sum up, titanium has strong corrosion resistance in acid and alkali. It can form oxide film in acid and alkali, but it is also conditional. I hope it can help you when using our materials.